Air cushion vehicles, commonly referred to by the trademark "Hovercraft.RTM." have been available for many years and have found many applications. These devices are also known as surface effect machines, ground effect vehicles and airborne surface vehicles. The term "air cushion vehicle" as used herein is intended to be synonymous with the other terms of art used for this class of vehicles. Since they move on a cushion of air, they can maneuver over water and most terrains. Air cushion vehicles can easily traverse terrains from asphalt to quicksand and can be used for tasks ranging from their well-known use as high-speed water transportation for people, vehicles and freight to ice or flood rescue and harvesting cranberries. Since an air cushion vehicle travels on a cushion of air it reduces or eliminates damage to ground surfaces. This makes it the transport of choice when avoidance of environmental damage is of concern. An air cushion vehicle also can travel on surfaces which are not otherwise easily traversable such as thin ice, swamps and marshes.
In general air cushion vehicles utilize an arrangement for producing a cushion of air under pressure beneath the vehicle so as to raise the vehicle a short distance from the supporting surface (the surface being traversed).
When it is raised from the supporting surface an air cushion vehicle is essentially floating on air and the propelling and steering means must be designed to take this into account. Many means have been provided in the past for providing thrusts in different directions to propel the vehicle forward, to brake forward motion or produce rearward motion and to turn the vehicle.
It can be seen that the basic requirements for propulsion are:
a) thrust generation to propel the vehicle at the speeds desired; PA1 b) generation of sufficient thrust to provide for acceleration and deceleration to ensure that the craft is maneuverable and safe to operate; and PA1 c) the ability to vector thrust to provide directional control. PA1 a) fins installed aft with rudders (yaw control only); PA1 b) differential thrust (with multi-propeller vehicles), including multiple swiveling propellers or ducted fans; PA1 c) swiveling bow thrusters; PA1 d) puff ports; PA1 e) propulsive force from the lift system with rudders installed in an outlet jet (and reverse thrust buckets for braking); PA1 f) skirt lining and PA1 g) surface contact devices such as retractable rods which create drag for braking and turning. PA1 a) one component acting on the direction of movement and thereby controlling the speed of the vehicle, and PA1 b) the other, acting perpendicular to the direction of movement and thereby controlling the sideways translation of the vehicle, commonly referred to as the "centripetal force". The magnitude of each component for a given thrust depends on the angle of the thrust with respect to the direction of movement of the vehicle. Lateral control of such vehicles relies on the perpendicular component to counteract the centrifugal force. Consequently air cushion vehicles in which yaw is the only operable directional control must be positioned at an oblique angle to their direction of movement to control lateral translation. PA1 complete control of the vehicle on hovering and in movement PA1 turning the vehicle without the need to position it at an oblique angle to its direction of movement PA1 use of a smaller power source because the relationship between the centripetal forces and the propulsive forces means that both do not need to be generated at the same time, which allows the use of a common power source to generate both forces PA1 the operator to use the total available propulsive thrust to counteract the centrifugal forces without the need to position the vehicle perpendicular to its direction of movement.
In smaller vehicles, integration of propulsion with the lift system conserves energy and minimizes the power system required. A combination of lift, propulsion and control in a single system affords a desirable design flexibility which affords simplicity and lower cost.
Control systems for yaw, side force and speed control usually have taken one of the following forms:
Aerodynamic control surfaces have been used in air cushion vehicles primarily for providing directional control by creating yaw moments. They have not been widely used because they are rather inefficient at the low speeds and large yaw angles which are often encountered in operation. This has led to placement of control surfaces in the slipstream of the propeller or fan used for propulsion, where their effectiveness is considerably improved. With a single propeller the rudder is commonly at the rear. With propellers and their respective engines mounted fore and aft on the vehicle, individual rudders may be located behind each propeller for steering.
In some skirt lift arrangements vanes in the peripheral skirt are controlled to change the direction of the air issuing therefrom. In others, a supply chamber for delivering pressurized air to a peripheral air curtain is provided with vanes on the sides and at the front and rear; and the vanes are controlled to produce movement of the vehicle in the desired direction.
If an air cushion machine is operating at the proper angle to the surface being traversed, horizontally or slightly nose-up, it is said to be properly trimmed. The trim or attitude of large air cushion vehicles is sometimes adjusted by changing inner skirt attachments to change the center of pressure of the air cushion and thus cause the vehicle to roll or pitch as desired. Longitudinal trim may also be adjusted by adding or shifting water or fuel (ballast) from one location to another. In small machines such devices are too cumbersome to include and the driver and any passengers move about to adjust trim. They must constantly shift their weight to assist the machine while accelerating, decelerating or banking into turns, in order to prevent the vehicle from nose-diving or becoming airborne.
Heretofore, it has been believed to be highly desirable for proper control of an air cushion vehicle to provide steering forces at both the front and the rear of the vehicle. This arrangement permits control in crosswinds and facilitates fairly quick turning about a reasonably small radius, but in practice this requires designing and operating a complex machine. Consequently a better method for control (steering) of air cushion vehicles would be highly desirable.
Many patents have appeared claiming improvements to air cushion vehicles. U.S. Pat. No. 3,173,509 to Wernicke et al. discloses a method for steering air cushion vehicles by directing air blasts through discharge ports placed at opposite corners fore and aft of the yaw axis of the vehicle. U.S. Pat. No. 5,042,605 to Moriwake utilizes two rudders placed downstream in the airstream of a ducted fan to effect steering.
U.S. Pat. No. 4,122,909 to Fair et al., U.S. Pat. No. 4,111,276 to Rapson et al., U.S. Pat. No. 3,409,103 to Tripp et al., U.S. Pat. No. 3,420,330 to Bliss and U.S. Pat. No. 3,384,197 to Bingham et al. disclose improved retaining skirts. Brake and stabilizer improvements are disclosed in U.S. Pat. No. 3,826,330 to Midolo et al. Improved cushioning is disclosed in U.S. Pat. No. 3,805,197 to Grignon et al. and U.S. Pat. No. 3,420,329 to Moore.